A persistent goal of many image forming devices is precise registration of images formed on media sheets. This may be particularly true of color printers using multiple color cartridges to create a single color image. In an effort to improve image registration, many image forming devices use an alignment mechanism to control the position and timing of media sheets traveling from various media sources, through the media path, and to the image forming location within the device. Thus, the image forming device relies on the alignment mechanism, which may include a variety of optical, electrical, or mechanical sensors, to know precisely where to form an image on the sheet.
As image forming devices are incorporated in smaller packages, rigid space constraints on the media transport components within the device create problems for devices having multiple feed sources. One example of this type of device is a laser printer with multiple media trays, a duplex path, and perhaps a manual feed path. Devices such as these may route media sheets from each of these sources through a common media path. As these devices become smaller, so too does the internal space used to align media fed from the multiple sources into the common media path.
A disadvantage of smaller device packaging is that, in general, more space and longer paths are desirable to accurately direct media sheets that are fed from multiple sources toward a common alignment point. Where sufficient space is available, the various media paths can be gradually merged to a common path so that sheets traveling in this common path may then repeatably arrive at a common alignment point. Further, with sufficient spacing, sheets arriving at this common alignment point may be sensed using a single leading edge sensor or other equivalent sensor. Thus, the timing of image processing and media transport events may be predictably determined. Thus, given sufficient space, the fact that media sheets arrive at the common alignment point from media paths converging from different directions and different approach angles may be nearly irrelevant.
Unfortunately, as image forming devices get smaller, alignment nips, rollers, and other alignment points move closer to the various media sources. Consequently, the distances previously relied on to align media from different sources get smaller and it has become increasingly difficult to provide consistent media sheet entry into these alignment points. Other factors such as media curl, media weight, and environmental conditions make it even more difficult to reliably control where the leading edge of a media sheet contacts the alignment point. For example, in an alignment nip formed at the contact surface between two registration rollers, the above factors may contribute to the leading edge of media sheets unpredictably striking either roll or both rolls simultaneously, leading to feed reliability problems such as skew, folding, or treeing.
Furthermore, the timings for each media source may not be consistent. With the sheets approaching the alignment point from varying angles and the leading edge of the sheets contacting the alignment point at different locations, the time that elapses between sensing a leading edge approaching the alignment point and passing of the leading edge through the alignment point may vary drastically. Thus, transport and image processing algorithms must accommodate this variation by implementing different feed times for the different sources or implementing large delay windows to account for the various feed times, neither of which is optimal.